The Postal Reorganization Act provides that the USPS operate out of it's own revenues. As a result it is "off budget" and there is no bill, rider or amendment to be considered in Congress that affects the budget.
As one wag once put it, there are entirely too many people in the ranks of Postal top management who are dedicated to the concept of the USPS as a privately operated universal monopoly.
And that's what happened here, and it's not the first time!